In a typical time division multiplexer (TDM) system, a transmitter samples pulse signals of relatively low pulse repetition frequency from various data sources or channels and interleaves them with one another to form an aggregate data stream that is transmitted by a high speed aggregate channel to a remote receiver. Ordinarily, the TDM transmitter inserts the signals representative of a single bit or a single character in a single time slot in the aggregate data stream and interleaves the signals from different channels on a bit-by-bit or character-by-character basis so that adjacent time slots contain signals from different channels. However, different size blocks of signals can be used if desired. At the receiver, the individual bits or characters are separated from one another and allocated to various low frequency data channels similar to those at the transmitter.
To permit proper decoding of the data stream at the receiver, the transmitter interleaves the signals from the various data channels in accordance with a fixed schedule which it repeats endlessly and the receiver uses the same schedule to decode the data stream. Each cycle of the schedule is called a frame or an aggregate frame. In addition to data signals, each frame ordinarily inclues synchronization signals called frame sync words and various control signals, both for individual channels and for the entire TDM system. Typically, the synchronization and control signals take up a small portion (less than 5%) of the total frame which is referred to as the overhead. To simplify the generation of the signals used to select the particular data channel from which a bit or character is to be transmitted, it is customary to sample the data channels in a fixed pattern which is repeated numerous times within each frame. Each such cycle of repetition is called a subframe.
In the prior art, a portion of each frame is typically assigned to the transmission of one or more frame sync words which are typically located in a contiguous portion of the frame near its beginning or end. At the beginning of data transmission, several frames are transmitted from the local station to the remote station solely for the purpose of establishing synchronization using these frame sync words. At the remote station a frame sync word detector scans the incoming bit stream, testing every bit and its contiguous bits for the presence of the predetermined bit pattern which constitutes a frame sync word. If this pattern is detected in one frame, the remote station then tests one or more successive frames for the presence of this pattern at the same place in the frame; and upon detecting this pattern as many times as required by the system protocol, acknowledges that it is in sync with the transmitting station. Typically, three such frames are required in the prior art in order to achieve synchronization and the length of each frame sync word is five to seven bits. However, because each frame may be thousands of bits long, the total time required to establish synchronization is appreciable. In addition, in some systems in which data signals or customer-originated EIA-type control signals are transmitted in the frame along with the frame sync word, the TDM systems has no control over the bit patterns that may occur in the frame. As a result, there is considerable likelihood that a series of contiguous data or control signals received at the remote station will have the same bit pattern as the frame sync word and that such a series of data or control signals will be mistakenly recognized at the remote station as the frame sync word. Such false synchronization will only be detected after the loss of considerable time where the bit pattern fails to repeat itself in subsequent frames. While such false sync problems can be avoided by clamping the data signals to exclusively one polarity, the time required to achieve synchronization is still quite long; and, when the data signals are unclamped, data or synchronization may be lost because of switching transients.